In a rotary type compressor, a vane is generally pressed against the outer peripheral surface of a roller by bias means of a hydraulic pressure, a spring or the like so as to maintain sealing tightness between these two members, thereby obtaining a high discharge pressure. The roller rotates eccentrically while it is constantly in contact with the vane. In order to increase a gas compression ratio in accordance with a demand for a higher performance of the compressor, a rotational speed of the roller must be increased. When the rotational speed of the roller is increased, wear of contact portions of the roller and the vane which are in constant with each other is remarkably increased. The roller rotates while it is in contact with the vane at its outer peripheral surface and with a cylinder wall at its both opposite end surfaces. Therefore, the roller is required to have properties not only of wear resistance per se but also of causing the vane and the cylinder which are mating sliding contact members not to wear.
Conventionally, such type of roller is made of cast iron by continuous casting, eutectic graphite cast iron, or Cu--Cr--system, Cu--Mo--system or Mo--Ni--Cr--system low alloy cast iron. Cast iron by continuous casting in particular is known to have a finer structure of the surface layer and a more excellent wear resistance property than cast iron produced by other casting methods. This type of material is disclosed in, for example, JP-B2-60-1943.
At present, the refrigerant used for compressors is CFC. As well known, when CFC is released into the atmosphere, it diffuses up to the stratosphere and is decomposed by irradiation of ultraviolet rays, thereby discharging chlorine which destroys the ozone layer. Such destruction of the ozone layer is globally considered as an environmental problem. Projects for totally abolishing use of CFC until the year 2000 have been formed, and development of an alternative refrigerant has progressed in each country.
As an alternative refrigerant, HFC containing no chlorine is the most promising. For example, 1,1,1,2 tetrafluoroethane (CH.sub.2 FCF.sub.3) known as R-134a may be raised. Use of such kind of refrigerant fluorocarbon involves the following problems when it is compared with use of the conventional CFC although it does not so much adversely affect the environment.
a) A lubricative property of the refrigerant is inferior. PA1 b) The compression ratio must be increased, and consequently, the load acting on the roller and the vane becomes greater. PA1 c) A hygroscopic property of the refrigerant is larger. PA1 d) A lubricative property of a lubricant used with the refrigerant is inferior. PA1 e) A hygroscopic property of the lubricant is larger. PA1 f) Wear of the sliding contact members such as the cylinder, the roller and the vane increases.
Especially, the problem of wear mentioned in Item f is important. Since the conventional CFC contains chlorine, it forms stable protective film (chloride) over the surfaces of the sliding contact members so as to provide the sliding contact surfaces with good wear resistance properties. On the other hand, HFC of an alternative fluorocarbon for overcoming the environmental problem contains no chlorine. Therefore, unlike CFC, such an advantageous effect of improving the wear resistance property can not be expected, and HFC involves a problem in practical use. Consequently, if the alternative fluorocarbon is used without changing kinds of the roller material, the roller wears heavily due to sliding contact with the vane, and scoring also occurs between the roller and the vane resulting in that durability as a practical compressor can not be obtained, although cast iron by continuous casting has excellent wear resistance properties.